This invention relates to a composite plastic and metal level measuring instrument which is inexpensive to manufacture yet is aesthetically pleasing and is durable enough to withstand the harsh environment of the work place.
In an effort to control level production costs, manufacturers have developed composite levels comprised of metal frames and plastic body portions. Wright U.S. Pat. No. 4,419,833 represents a level having one type of composite construction. Wright U.S. Pat. No. 4,463,501 discloses an indexing arrangement for securing plastic end plates to a metal frame.
In order to make these levels visually appealing, they are painted during the manufacturing process. The painting process requires the pretreatment of the level with a trichloroethylene solvent. The chemical pretreatment and the painting process are time consuming, expensive, and require careful controls. As a result, there is a need in the industry for a composite plastic and metal level which does not require painting.
In addition to the effect of painting on the appearance of the level, the milling process of the extruded metal frame also enhances the appearance of the level. After being extruded the metal frames are milled to ensure that the exposed or working surfaces of the level are smooth planes. Not only does the milling process provide planar working surfaces, it also leaves score lines on the exposed surface of the frame which enhance the level's appearance and minimize the visual effects of scratching. However, the milling process is expensive and time consuming to perform such that its successful elimination would result in substantial cost savings.
Another known composite level design is the Mayes SuperCede.TM. level which is constructed of a metal I-shape frame and a pair of single piece plastic cover plates that are arranged on opposite sides of the frame and are secured to one another by screws. As will be understood by one of ordinary skill in the art, the plastic material of the cover plates will shrink as it cures. Because the amount of shrinkage cannot be precisely predicted for the individual cover plates, there is a problem of accurately locating and securing the cover plates to the metal frame. The problem from the shrinkage of the plastic material would be especially acute in long levels where the length of a single piece cover plate would translate into greater amounts of shrinkage. While the Mayes level includes one piece plastic cover plates, it is a small 9 inch level where the shrinkage problem is negligible. The prior art does not disclose an indexing system for longer levels which can accurately position the cover plates on the frame regardless of the shrinkage of the plastic material.